1. The Field of the Invention
The present invention relates to novel trisubstituted symmetrical triazines, to processes for their preparation, and to their use as flame-retardant agents in synthetic resins. More particularly, the invention relates to derivatives of 2,4,6-trihydroxy-s-triazine, which contain ring-halogenated moieties.
2,4,6-Trihydroxy-s-triazine may exist in two limiting forms: the enol form (cyanuric acid) and the triketo form, known as isocyanuric acid-as well as in two intermediate mono- and di-keto forms. Isocyanuric acid exists in acidic solutions (below pH 6), and is completely enolized to cyanuric acid in strong basic solutions.
2. The Prior Art
Numerous mono-, di- and trisubstitued organic derivatives of cyanuric acid and of isocyanuric acid are known in the art, most of which are obtained from cyanuric chloride or by the trimerization of isocyanates, rather than from cyanuric acid. Attempts to obtain cyanurate esters directly or by means of alkali salts of cyanuric acid have been unsuccessful, and only the isocyanurate esters were so obtained [Smolin, E. M. and Rapoport, L., "The Chemistry of Heterocyclic Compounds", A. Weissberger Ed., Interscience, New York 1959, vol. 13, pp 1-48; Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed., Vol. 7, p. 400 (John Wiley & Sons, 1979)]. Instead of cyanurates, esters of isocyanuric acid are isolated when agents such as benzyl and alkyl chlorides are used.
The art is constantly searching for new and improved flame-retardant agents, to meet the ever increasing safety and health requirements and to improve the quality of the resulting plastic compositions and their processability. Halogen-containing compounds are commonly used for this application in one of two forms: as active fire-retardants or as simple additives to the polymeric material. An active fire retardant is defined as being chemically incorporated into the polymeric structure by formal chemical bonds. Simple additives are not chemically bonded but are simply dissolved and/or dispersed in the polymer matrix. The compounds of the invention are primarily directed to be used as simple additives to a variety of plastic materials.
Halogen containing fire (or flame) retardant materials (FRs), and particularly brominated FRs, often suffer from low thermal stability, which causes serious disadvantages. The release of acidic vapors, for instance, is usually accompanied by the discoloration of the plastic article during molding operations and further causes the corrosion of processing equipment. Additionally, polymeric compositions comprising halogenated FRs usually have enhanced sensitivity to UV radiation, which gives rise to accelerated product deterioration.
Because of compatibility problems, FR additives commonly are fugitive, viz., they are lost from the polymer matrix either during processing or during ageing of the finished product, or both. This can take place because of vaporization, bleeding of incompatible materials or the leaching of soluble additives. One such effect is known as "blooming", and may also render the product inacceptable because of the appearance of a powder of FR material on the surface of the object. This, of course, also diminishes the concentration of the active FR material within the polymeric matrix. These problems are particularly felt in connection with high performance plastics, i.e., those plastic materials intended for use under high stress conditions, such as high temperatures, which also require high temperatures for processing.
Employing polymeric FR additives to overcome fugitivity has not been entirely successful. Mark's Encyclopedia of Polymer Science and Technology [Vol. 7, Interscience Publishers, p. 19] cites among the serious disadvantages which limit the use of halogen-containing polymers as FRs, especially in thermoplastic molding compounds: (a) The halogen content of the polymer is usually of such a low level that massive amounts of polymer are required to obtain the desired flame-retardancy; (b) The low thermal stability of the halogen-containing polymer often limits processing to unrealistically low temperatures; and (c) Expensive processing techniques may be required for suitable blending of the two polymers.